feat: add optional background blur, align to shared texture pattern

Gaussian blur applied at texture load time when `background_blur` is
set in moonlock.toml. Refactored wallpaper loading from per-window
Picture::for_filename() to shared gdk::Texture pattern (matching
moonset/moongreet), avoiding redundant JPEG decoding on multi-monitor.

Cargo.toml

@@ -17,6 +17,7 @@ serde = { version = "1", features = ["derive"] }
 nix = { version = "0.29", features = ["user"] }
 zeroize = { version = "1", features = ["derive"] }
 libc = "0.2"
+image = { version = "0.25", default-features = false, features = ["jpeg", "png"] }
 log = "0.4"
 systemd-journal-logger = "2.2"